In general, oil is produced off-shore in deep water from a floating support that is anchored near to oil wells that are situated at the sea bottom, i.e. at depths that may lie in the range 1000 meters (m) to 2500 m, or even more. The floating support generally includes anchor means enabling it to remain in position in spite of the effects of current, wind, and swell. It generally also includes means for storing and processing oil and means for off-loading to off-loading tankers, which tankers call at regular intervals to off-load the production. Such floating supports are commonly referred to as floating production storage off-loading, commonly abbreviated to “FPSO”, which abbreviation is used throughout the description below.
Well heads are generally connected to such an FPSO via undersea pipes of the suspended catenary riser (SCR) type or of the hybrid tower type comprising:                a vertical riser having its bottom end anchored to the sea bottom and connected to a said pipe resting on the sea bottom, and having its top end tensioned by a float submerged in the subsurface and to which it is connected; and        a link pipe, generally a flexible link pipe extending between the top end of said riser and a floating support on the surface, said flexible link pipe taking up, where appropriate and under its own weight, the shape of a dipping catenary curve, i.e. going well below the float before rising up to the floating support.        
The entire production of crude oil is thus generally raised on board the FPSO where it is processed in order to separate the oil proper from the water, the gas, and possible sandy components. Once separated, the oil is then stored on board, the gas is washed and then delivered to gas turbines for producing the electricity and the heat needed on board, with any surplus being reinjected into the reservoir of the oil field so as to restore pressure in said reservoir. After being released of a sand in suspension, the water is finally rejected into the sea after thorough extraction of all particles of oil, or else it is likewise reinjected into the reservoir, with additional sea water taken from below the surface generally being added in order to achieve the flow rate required for injecting water into the reservoir. The extracted sand which is present in minute quantities only, is finally washed and then rejected into the sea.
A known method of separating the gas, water, and oil in crude oil that is commonly used in installations on land consist in using tanks of very large volumes, generally in the form of elongate cylinders, with the crude oil entering via one end and traveling along said tank for a duration of the order of 5 minutes (min) to 10 min, during which the various phases separate naturally under gravity before reaching the second end. The gas is then recovered from the top of the tank, the water and the sand from the bottom, and the oil from an intermediate portion. A very wide variety of separators of that type are in existence and they generally incorporate additional internal devices such as horizontal, vertical, or sloping screens, for the purpose of facilitating separation between the phases and preventing them from remixing at a subsequent stage.
Those separators operate at low pressure, e.g. 3 bars to 10 bars, and sometimes even at less than atmospheric pressure, in order to optimize the degassing of the crude oil. If it is desired to install that type of separator on the sea bottom, the tank must be capable of withstanding implosion under the effect of pressure that is substantially 100 bars, i.e. substantially 10 megapascals (MPa), per 1000 m of water depth. Thus, transposing such a tank for use at great or very great depth would require wall thicknesses of 100 millimeters (mm) to 150 mm in order to withstand implosion, and such sheet-metal work items would be very difficult and expensive to make and to install at great depths on the sea bottom.